Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels (which may include photodiodes and transistors) in a substrate to absorb (i.e., sense) radiation that is projected toward the substrate and convert the sensed radiation into electrical signals.
A back side illuminated (BSI) image sensor device is one type of image sensor device. These BSI image sensor devices are operable to detect light from the backside. Compared to front side illuminated (FSI) image sensor devices, BSI image sensor devices have improved performance, especially under low light conditions. However, traditional methods of fabricating BSI image sensor devices may still lead to certain shortcomings for BSI image sensor devices. For example, traditional BSI image sensors may require two types of isolation structures—for example a trench isolation as well as a metal grid—to provide sufficient isolation between adjacent pixels so as to reduce cross-talk. However, the need for having two different types of isolation devices result in more complicated (and lengthier) fabrication processes, which may increase fabrication costs. In addition, the two types of isolation structures need to be accurately aligned, and any misalignment may lead to substandard device performance and may even result in device failures.
Hence, while existing BSI image sensor devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.